Cryogenic cooling apparatus



F 17, 1970 R. R. HART CRYOGENIC COOLING APPARATUS Filed April 11, 1968 INVENTOR ROBEQT 1?- IT 0222; Mj

United States Patent O 3,495,419 CRYOGENIC COOLING APPARATUS Robert Revie Hart, Redditch, England, assignor to The Hymatic Engineering Company Limited, Redditch, England Filed Apr. 11, 1968, Ser. No. 720,660 Claims priority, application Great Britain, Apr. 14, 1967, 17,356/ 67; Oct. 24, 1967, 48,234/ 67 Int. Cl. F25b 9/02 US. Cl. 62467 2 Claims ABSTRACT OF THE DISCLOSURE A heat exchanger for a cryogenic cooling apparatus comprises a finned tube in the form of a frusto-conical coil, of about 90 total angle, through which refrigerant gas under pressure is supplied to an expansion nozzle adjacent the smaller end of the coil so as to expand and produce cooling by the Joule Thomson effect, whence the low-pressure gas flows back outwards between the fins around the tube.

This invention relates to cryogenic cooling apparatus, and is particularly, though not exclusively, concerned with small high speed coolers which are intended on each operation (or on their only operation) to produce low temperature, possibly of the order of that of liquid air, within quite a short time, for example a few seconds, and to maintain the temperature also for a relatively short time, perhaps of the order of a minute or two.

The invention is concerned with the type of cryogenic cooling apparatus including a heat exchanger formed by a finned tube wound in the form of a coil through which refrigerant gas is supplied to an expansion nozzle to expand and produce cooling by the Joule Thomson effect, whence the low pressure gas returns over the outer surface of the finned tube.

According to the present invention the coil is of frusto-conical form with its smaller end towards the expansion nozzle. For example it may have a total included angle of approximately 90.

In one form of the invention the coil is interposed between inner and outer frusto-conical surfaces and at least one filament is wound in the gap between the fins and one of these surfaces to divert the gas into closer contact with the tube and fins.

In a cooling apparatus ofv the type specified it can be shown theoretically that in order to reduce to a minimum the thermal capacity of the apparatus itself, it is desirable that the passageway for the low pressure exhaust refrigerant gas should be of continually increasing crosssection in the direction of flow. In this respect the frusto-conical form employed in accordance with the invention is a substantial improvement on a cylindrical helical coil. Little further advantage in this respect is obtained by increasing the angle of the cone, for example adopting a flat spiral, whilst on the other hand the employment of a limited angle of the order of 90 greatly facilitates (or indeed makes possible) the winding of a thread or filament of the terep'hthalate plastics materials, such as that sold under the trademark Terylene, in the concave-sided triangular space between the fins and the inner and outer frusto-conical surfaces. Such a winding is well known in connection with helical heat exchangers and serves to deflect the gas so as to pass into intimate contact with the fins and tube.

In one form of the invention a portion of the tube at its warm end is in good heat exchange relationship with a heat sink, such as a portion of the body of the apparatus of substantial mass. In particular, where the tube is of 3,495,419 Patented Feb. 17, 1970 narrow bore and emerges from a much more substantial tube and has an unfinned portion between the body and the finned portion, the unfinned portion is preferably in good heat exchange relationship with the heat sink by being soldered to it, for example by being embedded in a substantial body of solder.

The desirability of this feature arises from the fact that when an apparatus of the type specified is in operation, almost the whole drop of pressure occurs across the expansion nozzle at the cold end of the heat exchanger. Accordingly when the apparatus is first put into action, the charging of the tube with high pressure gas represents compression, and if the gas retained all the heat of compression its temperature could rise to quite a high value, for example of the order of 1000 K. In the finned part of the tube forming the heat exchanger the heat is rapidly absorbed and the resultant temperature of the gas and tube may be only a few degrees higher than the initial temperature, the heat being quickly removed by the low pressure exhaust gas.

On the other hand the length of tubing between the inlet manifold and the finned portion constituting the heat exchanger will rise in temperature and this heat can only be removed by the high pressure gas, thereby reducing the efiiciency of the apparatus. This is overcome in accordance with a preferred feature of the invention by thermally anchoring the portion of the tube between the heat exchanger and the inlet to the body of the cooler.

The invention may be put into practice in various ways, but one specific embodiment will be described by way of example with reference to the accompanying drawing, in which the single figure is a longitudinal section of a cryogenic cooler working on the Joule Thomson principle.

The invention is shown as applied to a cryogenic cooling apparatus intended to reduce the temperature of a small component or load to a value of the order of K. in a few seconds, and retain it at that temperature for a minute or two, for which purpose it is supplied with a charge of refrigerant gas such as air, oxygen, nitrogen, or argon under high pressure.

The cooler comprises an outer annular body 10, which it will be assumed is arranged with its axis vertical, of which the lower part 11 contains or contacts the component to be cooled and which has in it a central bore 12, affording a collecting chamber for a supply of the liquid refrigerant. Above the collecting chamber the bore widens out to afford a frusto-conical recess 15 of approximately included angle. Within the frusto-conical recess, but spaced from it, is a frusto-conical plug 16 conveniently in the form of a hollow chamber 17 of stainless steel the top of which is formed by a mushroom shaped coupling member 18 the stem 19 of which is formed as a coupling 20 for connection to the gas inlet and contains a filter 21. A thin finned tube 25 is wound in a coil on the frustoconical surface of the plug. Its innerend 26, deprived of fins, extends diametrically across the plug and is secured to it and blocked at its extreme end whilst a small hole is formed in its wall to constitute an expansion nozzle. Thence the tube is wound in a frusto-conical coil on the corresponding surface of the plug with a thread or filament 30 of Terylene wound in the gaps between the fins of adjacent convolutions and the adjacent surface of the plug. A similar winding 31 of Terylene thread or filament is wound in the corresponding outer grooves between adjacent convolutions of the finned tube, and the plug is then inserted into the conical recess in the outer body. The outer end of the coiled tube extends radially into the coupling by which it is connected to the gas supply.

This unfinned length of tube is embedded in a substantial mass 35 of solder uniting it and thermally anchoring it to the body of the apparatus. A cover 3 6 fits round the stem 19 and the body 10', to which it is sealed by rings 3-7 and 38 respectively, and is provided with an outlet connection 3 9 for the low pressure gas.

The operation of the device is in accordance with known practice. Thus when the gas under high pressure is released to the inlet it flows spirally through the tubular coil to its inner end Where it escapes from the expansion nozzle, and thence flows outwardly in a diverging path through the gap between the conical recess and the conical plug in intimate contact with the fins and tubes so as to cool the incoming gas. In a few seconds the refrigerant gas begins to be liquified, and a supply of the liquid refrigerant collects in the collecting chamber so as to maintain the temperature within the required range for a further period after the supply of gas is exhausted.

The mass 35 of solder connected to the coupling member 18 functions as a heat sink as already described to prevent undue rise of temperature of the unfinned part of the tube 25 when the gas in it is initially compressed.

What I claim as my invention and desire to secure by Letters Patent is:

1. A cryogenic cooling apparatus including a heat exchanger formed by a finned tube wound in the form of a frusto-conical coil having a total included angle of approximately 90, an expansion nozzle connected to the outlet end of the tube, means for connecting the inlet end of the tube to a source of gas under pressure at a temperature below its conversion temperature to effect expansion at the nozzle and cooling by the Joule Thomson efiect, inner and outer frusto-conical surfaces between which the coil is interposed to direct the low pressure gas from the nozzle back over the outer surface of the finned tube, and at least one filament wound in the cap between the fins and one of the said frusto-conical surfaces to divert the gas into closer contact with the tube and fins.

2. Apparatus as claimed in claim 1 in which the tube is of narrow bore and emerges from a much more Substantial body and has an unfinned portion between the body and the finned portion and the unfinned portionis in good heat exchange relationship with a heat sink formed by a portion of the body by being soldered to it.

References Cited UNITED STATES PATENTS 3,018,643 1/1962 Evers 62-514 XR 3,095,711 7/1963 Wurtz 62-514 3,364,697 1/1968 Garrett 62-5l4 MEYER PERLIN, Primary Examiner 

